(Invited) Achieving Full Utilization of Na x V 2 (PO 4 ) 3 (0 < x < 3) NASICON Material: A New Platform for Polyanion Electrodes

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More than 40 years ago, Hong and Goodenough developed the “Natrium Superionic CONductor” (NaSiCON), which reached mS cm –1 Na-ion conductivities at room temperature. 1,2 The replacement of Zr with the first row of transition metal forms NaSiCONs, which are considered promising electrodes for sodium-ion batteries because of their 3D open-framework crystal structure, resulting in high cyclability and fast rate capability and minimal lattice variation at different states of charge. 3–7 In this talk, we report on new NASICON-type single-phase Na x V 2 (PO 4 ) 3 compositions (1.5 ≤ x ≤ 2.5) that had never been reported before, obtained from an innovative and straightforward synthesis route informed by computational simulations. 8,9,10 Guided by predicted phase diagrams from first-principles calculations, the chemically c-Na 2 V 2 (PO 4 ) 3 phase was synthesized by annealing an equimolar mixture of Na 3 V 2 (PO 4 ) 3 and NaV 2 (PO 4 ) 3 exhibits a new distribution of sodium ions with an occupancy of the Na(1) site of only 0.66(4) whereas that of the electrochemically-obtained e-Na 2 V 2 (PO 4 ) 3 (from Na 3 V 2 (PO 4 ) 3 ) is close to 1.15. Unlike conventional Na 3 V 2 (PO 4 ) 3 , when used as positive electrode materials in Na-ion batteries, the new Na x V 2 (PO 4 ) 3 compositions identified lead to unusual single-phase Na + extraction/insertion mechanisms with continuous voltage changes upon Na + extraction/insertion. 9,10 We demonstrate that the average equilibrium operating voltage observed upon Na + deintercalation from the single-phase Na 2 V 2 (PO 4 ) 3 is significantly increased up to an average value of ~3.70 V vs. Na+/Na (thanks to the activation of the V IV /V V redox couple) compared to 3.37 V vs. Na + /Na in conventional Na 3 V 2 (PO 4 ) 3 , thus leading to an increase in the theoretical energy density from ~396.3 Wh/kg to ~458.1 Wh/kg. Furthermore, never reported before NASICON-type compositions, such as Na x V 2 (PO 4 ) 3 with 0 ≤ x < 1 were obtained by electrochemical and chemical Na + deintercalation from the new c-Na 2 V 2 (PO 4 ). 9,10 References Y. P. Hong, Crystal structures and crystal chemistry in the system Na 1+x Zr 2 Si x P 3-x O 12 , Mater. Res. Bull. , 11 , 173 (1976). B. Goodenough, H. Y. P. Hong and J. A. Kafalas, Fast sodium (1+) ion transport in skeleton structures, Mater. Res. Bull. , 11 , 203 (1976). Masquelier and L. Croguennec, Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries, Chem. Rev. 113 , 8, 6552 (2013). N. Chotard, G. Rousse, R. David, O. Mentré, M. Courty, and C. Masquelier, Discovery of a Sodium-Ordered Form of Na 3 V 2 (PO 4 ) 3 below Ambient Temperature, Chem. Mater. , 27 , 5982 (2015) Singh, Z. Wang, S. Park, G. S. Gautam, J.-N. Chotard, L. Croguennec, D. Carlier, A. K. Cheetham, C. Masquelier, and P. Canepa, A chemical map of NaSICON electrode materials for sodium-ion batteries, J. Mater. Chem. A , 9 , 281 (2021). Park, Z. Wang, Z. Deng, I. Mong, P. Canepa, F. Fauth, D. Carlier, L. Croguennec, C. Masquelier, and J.-N. Chotard, Crystal Structure of Na 2 V 2 (PO 4 ) 3 , an Intriguing Phase Spotted in the Na 3 V 2 (PO 4 ) 3 –Na 1

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